Nasa's Dawn spacecraft approaches protoplanet Vesta

After almost four years of space travel, Nasa's Dawn probe
is approaching the pumpkin-shaped protoplanet Vesta, which is
floating amidst the asteroid belt.

With an average diameter of just 330 miles -- about 24 times
less than Earth's -- Vesta is too puny and doesn't have a large
enough gravitational tug to retain much of an atmosphere.
Otherwise, it has mostly the same features and structure as Mercury, Venus, Earth and Mars.

Nasa launched Dawn in September 2007, and pointed it towards
the crowded asteroid belt between Mars and Jupiter. It will arrive in July 2011, and descend towards the
planet in the coming months. By November it will be just 125 miles
above the surface, and in January 2012 we should get the first
high-resolution images of the surface.

The spacecraft is kitted out with tools and scientific
instruments to answer some key questions about the protoplanet. It
has a high-quality camera, a visible and near-infrared spectrometer for identifying minerals, a gamma ray and neutron
spectrometer for element (and possibly water) hunting and a radio
signal device to determine Vesta's gravity.

So, what are those key questions? "One of them is why Vesta
is so bright?" asks Christopher Russell, a UCLA professor of
geophysics and space physics and the Vesta mission's principal
investigator.

Vesta is very bright -- closer to the Earth, which reflects 40 percent of sunlight with its snow and clouds, than
the Moon, which only reflects ten percent. What is on the surface
of this shapely protoplanet that's causing all that light to be
reflected? "We'll find out," declares Russell.

The team also wants to find out about Vesta's core. Russell
expects that the body's interior is layered like Earth's, with a
crust, a mantle and an iron core. He wants to find out more about
the interior, and find out how large that iron core is. The
orbiting craft will also map the entire surface.

We already know a fair bit about Vesta. During its
4.5-billion-year life, it's been bombarded with meteorites, and has
sent shards and rocks spiralling off into space. A lot of them make
their way to Earth: about one in every 20 meteorites that falls on the surface of the Earth come from
Vesta.

Dawn is the first scientific spacecraft to use ion thrusters.
It has ion engines and xenon fuel to sharply cut travel costs by
going farther on less fuel. With ion propulsion systems, spacecraft
can travel about ten times as far as chemical-based rockets, albeit
with relatively low acceleration.

Dawn will orbit Vesta for a year, while it takes photos,
carries out investigations and sends back data to Earth. It will
then move onto another asteroid belt object: the "dwarf planet" Ceres.